1. Field of the Invention
The present invention relates to a magnetic position sensor, and in particular to a magnetic position sensor that is capable of detecting positions in a short cycle with accuracy. The present invention further relates to a position detecting method by the magnetic position sensor.
2. Description of the Related Art
The applicant has developed a magnetic position sensor that detects a magnetic mark such as a magnet with a coil array (for example, Patent Literature 1: JP 2008-209393A). This position sensor has a presumption that when the coil array is arranged in parallel to a magnetic pole surface, the magnetic flux density changes in a sine wave shape in the longitudinal direction of the coil array. However, actually, there is a variation in the distance between the coil array and the magnetic pole surface. When the distance deviates from a normal distance, the strength of the magnetic flux density also deviates from the sine wave so as to be approximated to a triangle wave or a trapezoid wave, which causes a detection error.
The above-described position sensor has another problem that it is difficult to detect a position at a high speed, that is, in a short repetition cycle. In the above-described position sensor, an alternating current is applied to the coil array, and the position at which the phase of the voltage, the current, or the like that is applied to the coil is zero is detected. Because it is difficult to increase the frequency of a coil current, the frequency of the alternating current restricts the number of times of detection per unit time.
A position sensor of Patent Literature 2 (JP 2007-178158A) is known as a position sensor in which instead of coils, magnetic detection elements such as hall elements are used. This position sensor detects the density of a magnetic flux from a pair of magnets with an array of magnetic detection elements, and detects a point at which the magnetic flux density is zero, that is, an intermediate point of the pair of magnets. Assuming that this point is referred to as a “zero crossing point”, the magnetic detection elements to both sides of the zero crossing point have outputs whose signs are inverted with respect to each other, and the magnetic flux density changes substantially linearly. Accordingly, a straight line that approximates a distribution of outputs of magnetic detection elements in the vicinity of the zero crossing point is obtained by a least squares method, and a point at which the value of the straight line is zero is set as the zero crossing point. In order to obtain the zero crossing point with accuracy and realize a certain level of measurement range, it is necessary to arrange a large number of magnetic detection elements into an array. However, it takes a processing time for scanning outputs of the large number of magnetic detection elements to obtain the position of the zero crossing point.